Heavy metals such as copper, nickel, zinc and cobalt are nutrients that play important biological roles at low concentrations. However, they become toxic to the organisms at high concentrations. Other heavy metals such as mercury, cadmium and lead have no function in the living organisms and are highly toxic at low concentrations.
The maximum acceptable concentrations established by the Ministry of the Environment of Quebec (Ministère de l'Environnement du Québec, 1998, politique de protection des sols et de réhabilitation des terrains contaminés) for mercury, cadmium and copper are 0.2, 1.5 and 40 μg/g respectively.
The heavy metals present in contaminated soils and waters come from natural and anthropogenic sources either by mining or other activities such as the production of electricity using fossil fuels.
The presence of heavy metals such as mercury in domestic waters, industrial waste waters, and in agricultural soils has increased in recent decades and is a major cause of contamination. This is of increasing concern due to the high toxicity of mercury for living cells.
Mercury contamination of water occurs mainly by atmospheric deposition such as rainfall and runoff from industrial waste waters where mercuric ion Hg2+ is the main form of mercury.
The deposition of mining waste rich in minerals, copper smelters, incineration emissions, and combustion of fossil fuels are important anthropogenic activities that contaminate large soil areas with toxic elements. According to the Environmental Protection Agency (EPA) from U.S.A., in 2007 gold mining accounts for 18% of anthropogenic mercury emissions to the soil.
The ionic mercury can be methylated by microorganisms to produce methylmercury (MeHg+). Methylmercury is the most toxic mercury (II) species and represents a public health problem. The bioaccumulation and biomagnification of mercury in the food chain could be a risk for human health. The risks associated with mercury contamination have increased the restrictions to this heavy metal in liquid industrial wastes, where the maximum acceptable concentration in Europe is ≦50 ng/g.
Mercury species are toxic. The exposure to mercury could cause severe neurological diseases and the death. Organomercurial compounds are the most toxic chemical forms of Hg (II).
The mining activities and gold extraction are main sources of mercury contamination in soils. Gold mining in large-scale and artisanal mining, which is widely distributed in the world use mercury to extract gold. As a consequence, mercury may pollute soils, affecting human health and the environment.
Cadmium toxicity is caused by the decrease of glutathione and its interaction with sulfhydryl groups of proteins. Exposure to cadmium can cause chronic lung diseases, and kidney diseases, hypertension and bone disorders. Cadmium affects the calcium metabolism. Cadmium inhibits the activation of vitamin D and decreases the absorption of calcium and consequently the bone mineralization.
Cadmium moves easily in the environment from soil to plants by root absorption, thus entering the food chain affecting the human health.
Copper is an essential micronutrient widely distributed in nature. Copper is an essential component of many enzymes such as oxidases. The acute or chronic ingestion of copper salts may cause hepatic necrosis and the death. Copper contamination comes mainly from mining activities and the application of pesticides, fungicides and algaecides. As a result of mining activities, agricultural soils have been contaminated with significant levels of copper. Copper moves from soil to animals and humans through the food chain.
Heavy metals can be removed from contaminated sites by physicochemical processes such as ion exchange by columns, adsorbents such as activated carbon, chemical precipitation, filtration processes, etc. These processes are not selective and require further treatment that involves expensive and regeneration processes. Generally, physicochemical processes may involve the production of more toxic compounds either by concentration processes or by formation of new products by applying chemicals for precipitation, coagulation and flocculation.
Mercury removal by biological methods such as bacterial bioremediation is an attractive alternative to physicochemical treatments. This alternative has shown to be robust with relatively low cost on an industrial scale when it has been applied to industrial waste liquids from chloro-alkali manufacturing. The biological removal of mercury is highly selective and efficient and can minimize the final volume of the pollutant for final disposal.
Microorganisms such as bacteria are involved in the global mercury cycle by reducing chemical forms of Hg(II) (Hg+2, MeHg+) to the metallic form Hg0. Reduced mercury (Hg0) is less soluble in aqueous systems and therefore, is less bioavailable. Metallic mercury is the less toxic form of all mercury species. The biotechnological application of mercury removal from contaminated water and soil includes the biotransformation of the toxic mercury (II) in the reduced form and the collection of mercury from the gas phase in an oxidizing solution. Mercury is re-oxidized and then could be precipitated with alkali or sulfide.
Reduction of mercury (II) forms to elemental mercury is widely distributed in Gram-positive and Gram-negative bacteria. Genes responsible for metal uptake and reduction are organized in operons present in plasmids and transposons. The merRTPABD cluster is a typical mer operon in Gram-negative bacteria, which confers resistance to mercury compounds. Mercury induces the expression of structural genes merTPABD. The expression of the mer genes is regulated by a transcriptional regulator encoded by the merR gene. MerR is a transcriptional regulator of the mer operon which acts as a repressor or activator in the absence or presence of mercury, respectively. MerD is a protein that is synthesized by the cell when the mercury has been completely removed from the cytoplasm and acts as a distal regulator. MerP is a periplasmic protein that captures extracellular mercury and transfer it to the MerT membrane protein, which delivers Hg(II) to the cytosolic protein MerA (mercuric reductase) that enzymatically reduces the ionic mercury to the metallic state. MerB is an organomercurial lyase that catalyses the protonolytic cleavage of carbon-mercury bonds in organomercurial compounds releasing Hg (II) for the reduction by MerA. The mer operon lacking the merB gene is classified as narrow-spectrum operon, and confers only a low resistance to inorganic mercury salts and not resistance to organomercurial compounds. The mer operon that contains the gene merB is classified as a broad-spectrum operon that confers resistance to both inorganic and organic mercury species. In a recent study, a plasmid (pTP6) was exogenously isolated from a site contaminated with mercury in the river Nura, Kazakhstan. The pTP6 plasmid carries a complex mer transposon as sole accessory element (Smalla K. et al. en “Increased abundance of IncP-1 beta plasmids and mercury resistance genes in mercury-polluted river sediments: First discovery of IncP-1 beta plasmids with a complex mer transposon as the sole accessory element”. Appl Environ Microbiol 2006, 72:7253-7259). The plasmid pTP6 is an IncP-1β type plasmid that has acquired a transposon (Tn50580) encoding broad-spectrum mercury resistance genes. The mercury resistances genes that are present in Tn50580 of plasmid pTP6 are merR1, merT, merP, merA, merG, merB1: merR2, merB2, merD2 and merE.
Mercury-contaminated sites by mining activities often are polluted with other heavy metals such as cadmium and copper. For bioremediation of heavy metals such as mercury, cadmium and copper, a wide range heavy metal-resistant microorganism should be used.
The strain of this invention is useful for this technical challenge. The bacterium Cupriavidus metallidurans strain MSR33 is useful for the removal of mercury as well cadmium and copper from contaminated sites. Cupriavidus metallidurans strain MSR33, is a recombinant bacterial strain generated by incorporation of the natural plasmid pTP6 into the native strain Cupriavidus metallidurans CH34. This plasmid confers to the native strain a broad-spectrum mercury resistance and also an increased cadmium resistance, maintaining the copper resistance. Plasmid pTP6 is a natural plasmid that contains mercury resistances genes in the transposon Tn50580 conferring a broad-spectrum resistance. The mercury resistance genes that are present in Tn50580 of plasmid pTP6 are merR1, merT, merP, merA, merG, merB1: merR2, merB2, merD2 and merE.
Cupriavidus metallidurans (formerly Wautersia metallidurans, Ralstonia metallidurans, Alcaligenes eutrophus) strain CH34 is a native metal-resistant bacterium isolated from a zinc decantation tank (Mergeay et. al. in “Alcaligenes eutrophus CH34 is a facultative chemolithotroph with plasmid-bound resistance to heavy metals”. J Bacteriol 1985, 162:328-334, von Rozycki et. al. in “Cupriavidus metallidurans: evolution of a metal-resistant bacterium”. Antonie Van Leeuwenhoek 2009, 96:115-39). Cupriavidus metallidurans CH34 has two plasmids: pMOL28 (171 kb) and pMOL30 (234 kb), containing several genetic determinants of heavy metal resistance. The genes involved in Hg(II), Co(II), Cr(VI) and Ni(II) resistance are located in a 34-kb region on pMOL28 and genes involved in Hg (II), Cd (II), Cu (II), Ag (I), Co (II), Pb (II) and Zn (II) resistance are located in a 132-kb region on pMOL30. Each plasmid possesses mercury determinants of narrow spectrum (i.e. have only a slight resistance to inorganic mercury). The operons merRTPAD are present in the transposons Tn4378 (pMOL28) and Tn4380 (pMOL30). Mercury bioremediation of contaminated sites using Cupriavidus metallidurans CH34 has not been reported.